Various types of structures are used to extract oil and gas from offshore reservoirs. Most of these structures include a horizontal working platform that is supported at a safe distance above the water's surface by a support device. These support devices include floatation devices that are held in place with anchors, temporarily installed submergible devices, and permanently installed submergible devices. A typical permanently installed support device for an offshore platform consists of long piles that are driven into the underwater floor using a jacket.
The jacket of an offshore platform is that portion of the platform that rests on the underwater floor and through which piles are driven to permanently support the entire platform. It includes hollow pile sleeves that serve as guides for driving the piles and that assure that each pile will be properly placed. In addition to the pile sleeves, the jacket includes many horizontal, vertical, and diagonal supports that provide support for the piles against lateral loads.
Many offshore areas have very soft, unconsolidated underwater floors, which present challenging problems with regard to jacket installation. In particular, after the jacket has been lowered to the sea floor, it is often very difficult to perform the pile driving operations since the jacket tends to sink into the soft mud around the area of the jacket surrounding the pile that is currently being driven. This problem is typically resolved by attaching mudmats to the bottom of the jacket.
Mudmats are used to support the jacket structure of an offshore platform during installation of the platform. In particular, the mudmats have a large area and are thereby able to distribute the load of the jacket over that large area. This allows the jacket to stand on the soft underwater floor and remain stable during the pile driving operations, which typically last 3 to 4 days, but may last as long as three weeks during problematic installations. Mudmats, thus, are an integral part of the mudline framing plane, which is the lowest level of framing in the jacket.
Conventional steel mudmats consist of the major steel pipe members of the mudline framing plane, the mudmat skin or planking, and the integral mudmat framing beams (which support the mudmat planking and which arc typically steel wide flange beams, but may also be steel pipe). Conventional mudmat planking is typically flat steel plate, stiffened steel plate, or crimped steel plate or sheet piling.
There are several disadvantages to using conventional steel skinned mudmats. First, the steel skin for the mudmats is heavy (typically 15.3 psf in air and 13.3 psf submerged). Second, since conventional mudmats are made of steel, they must be protected cathodically. This need for cathodic protection is present despite the mudmats' short useful life since conventional mudmats continue to draft from the platform's cathodic protection after their usefulness has ended. Third, due to their need for cathodic protection, conventional steel mudmats require the placement of additional anodes even when the mudmats are no longer serving any useful purpose. Each additional anode increases the cost of the mudmat system by about $1500 and increases the weight of the jacket by about 900 pounds.
Thus, there is a need for a mudmat that is made of a non-corroding material and hence does not require as much cathodic protection. Further, such mudmats should preferably have a high flexural strength and be lightweight.
Accordingly, it is an object of the present invention to provide a mudmat that does not require as much cathodic protection, has a high flexural strength, and that is lighter than conventional mudmats.